BACnet (Building Automation and Control Networks, ASHRAE 135)
BACnet is the open communication protocol for building automation and control networks, developed by ASHRAE and standardized as ANSI/ASHRAE Standard 135 and ISO 16484-5. BACnet defines an object model (analog input, analog output, binary input, binary output, schedule, calendar, trend log, notification class, and 30+ other object types), a service model (read property, write property, subscribe COV, alarm, file transfer), and a network architecture supporting multiple physical layers (BACnet/IP, BACnet/SC, MS/TP, ARCNET). BACnet is the dominant BAS protocol in commercial buildings worldwide.
What is a BACnet object?.
A BACnet object is an addressable container that represents a sensor, an actuator, a setpoint, a schedule, a trend log, an alarm, or other building-control entity. Each object has an object identifier (object type plus instance number) and a set of properties (current value, units, description, present-value priority array, alarm thresholds, COV increment). Standard object types include analog input (AI: a sensor reading), analog output (AO: a setpoint or commanded value), binary input (BI: a status), binary output (BO: a command), multistate value, calendar, schedule, notification class, trend log, file, device. Each device exposes an inventory of objects; the BAS configuration tool reads and writes object properties to commission and operate the system.
What is the BACnet priority array?.
Output objects (AO, BO, multistate output) support a priority array with 16 priority levels. Each priority level can hold a commanded value; the actual output is the highest-priority non-null value. Priority 1 is manual life-safety (rare); priority 8 is manual operator override; priority 16 is the lowest (typically program control). The priority array allows multiple programs and operators to command an output without overwriting each other: each writes to its own priority level, and the priority array resolves conflicts. Operator overrides at priority 8 win over program control at priority 16.
What is COV (Change of Value)?.
COV (Change of Value) is the BACnet notification mechanism where a device sends an unsolicited update when an object property changes by more than a configured increment. COV reduces network traffic versus polling: the BAS supervisory tool subscribes to COV notifications from each sensor, and the sensor only sends an update when its value changes significantly. Each subscribing client controls the COV increment per subscription. COV notifications include the new value and an aging time so the supervisory tool can detect a sensor that has gone silent.
Frequently asked.
What physical layers does BACnet support?
BACnet supports several physical layers: BACnet/IP (Ethernet, the most common in modern systems), BACnet/SC (Secure Connect, encrypted IP for cybersecurity), MS/TP (Master-Slave/Token-Passing, RS-485 serial for field-bus runs), ARCNET (legacy, rarely deployed in new systems), PTP (Point-to-Point, for dial-up modem applications). Most modern installations use BACnet/IP for the backbone and MS/TP for field devices (VAV terminal controllers, fan coil controllers).
What is BTL?
BTL (BACnet Testing Laboratories) is the third-party certification program operated by BACnet International that tests devices for conformance to ASHRAE 135. BTL-listed devices have been independently verified to support the BACnet object types and services they claim. Specifications often require BTL-listed equipment to ensure interoperability.
Can BACnet integrate with other BAS protocols?
Yes, through gateways. BACnet gateways translate between BACnet and Modbus, LonWorks, KNX, M-Bus, OPC UA, MQTT, and proprietary vendor protocols. Gateway integration is common in legacy buildings where some equipment uses non-BACnet protocols. The gateway exposes the non-BACnet equipment as BACnet objects to the BACnet network.
What is BACnet/SC?
BACnet/SC (Secure Connect) is the encrypted IP transport for BACnet, standardized in 2019. It uses TLS over WebSockets and provides authentication, encryption, and integrity protection. BACnet/SC addresses the cybersecurity concerns with traditional BACnet/IP (which uses unauthenticated UDP) and is increasingly required for new installations in critical-infrastructure buildings.